Thiosulfate copper plating

ABSTRACT

A novel electroplating bath suitable for electrodepositing copper on steel which comprises an aqueous solution of soluble copper, an alkali metal thiosulfate, an alkali metal sulfite or bisulfite, and a heterocyclic nitrogen compound such as nicotine, 3-amino-1,2,4-triazole, etc. During the electroplating process, the bath is subjected to aeration. The steel is cleaned and cathodically activated prior to plating.

United States Patent Leo J. Slominski Bristol;

Adela Landau, Watertown, both of Conn. 831,983

June 10, 1969 Nov. 2, 197 l MacDermid Incorporated Waterbury, Conn.

Inventors Appl. No. Filed Patented Assignee THIOSULFATE COPPER PLATING44, 106, 123,34, DIG. 2; 106/1; 117/1305 ABSTRACT: A novelelectroplating bath suitable for electrodepositing copper on steel whichcomprises an aqueous solution of soluble copper, an alkali metalthiosulfate, an alkali metal sulfite or bisulfite, and a heterocyclicnitrogen compound such as nicotine, 3-amino-l,2,4-triazole, etc. Duringthe electroplating process, the bath is subjected to aeration. The steelis cleaned and cathodically activated prior to platmg.

THIOSULFATE COPPER PLATING This invention relates to a process forelectroplating copper on steel and to a novel electroplating bath. Moreparticularly, this invention relates to a thiosulfate-containingelectroplating bath containing therein a heterocyclic nitrogen compoundand to an electrolytic process for plating copper on steel-employingaeration.

A number of processes are known in the art for the plating of copper onsteel. The cyanide copper process is almost universally used forelectroplating copper for this purpose. Although heretofore the cyanideprocess has been the only copper-plating system producing a coppersystem coating or deposit which adhered directly to steel, it sufi'ersfrom a number of distinct disadvantages, namely (1) the cyanide solutionbeing toxic presents extreme waste disposal problems, (2) the specificresistivity of cyanide solutions is high, (3) during the electroplatingprocess the cyanide is oxidized to carbonate at the anodes and theaccumulation of carbonate in the solution limits its life and,additionally, requires periodic treatment for removal, (4) the cyanidesolution is easily contaminated by chromium and as little as 0.001 molarconcentration of chromate ion in the solution causes misplates andirregular deposits, (5) the plated specimens exhibit low specularbrightness, and (6) nodular deposits built up during the plating processprevent the formation of heavy thicknesses such as required forelectroforming operations. The acid copper sulfate bath has been widelyused for the deposition of copper since plated specimens produced bythis method have high specular brightness (i.e., mirror-typereflectivity). lt, likewise, suffers from several disadvantages inthat 1) it is not possible to plate directly on steel utilizing thismethod since immersion deposition occurs and (2) the process requiresexpensive, acid-proof equipment. In through-hole printed circuittechnology the use of the pyrophosphate copper-plating process isfavored since the specular brightness approaches that of copper platedfrom acid copper sulfate baths. However, this process also suffers fromcertain disadvantages. It is not possible to plate copper on steel froma pyrophosphate bath since the steel exhibits no adhesion to such copperplating, there is a strong ammonia odor present during the platingoperations and the pyrophosphate hydrolyzes to orthophosphateproportionately with time, high temperature and high pH which limits thesolution life and, although this may be controlled by dilution, theresult is a substantial waste in plating materials.

The objects of this invention are: (l) to eliminate the use of cyanidesolutions in copper plating on steel with its toxicity and disposalproblems, (2) to provide a method of forming an adherent coating ofcopper directly to steel in the plating process, (3) to provide a coppercoating with an improved 7 specular brightness, )4) to provide achemically and electrolytically stable copper-plating solution, and (5)to eliminate the problem of chromium contamination.

Thompson, in U.S. Pat. No. 58,037 (1866), discloses a process forplating copper on steel in which the steel specimen is first coated withpure ironand then a coating of copper is deposited from a solution ofhyposulfite of soda saturated with hydrated oxide of copper. The resultwas a copper-plated steel article of very poor copper appearance, thedeposition rate was low, the solutions employed were unstable and therewas no adhesion directly to steel. Gernes in Trans. Elecrrochem. $00.,77, I77 (I940) investigated the plating of copper on steel from athiosulfate bath. Gernes, who employed a highly purified grade ofcuprous chloride, was not able to prepare bright copper deposits onsteel even though he used low current densities and, in addition,considerable decomposition occurred in all his baths.

Plating processes set forth in the art employing sodium thiosulfate andcuprous chloride or other copper salts have yielded copper-plated, steelspecimens in which the copper coating was a grayish to brown color, andthe copper was laid down as rough, powdery, noncoherent, nonadherentdeposits. Likewise, the thiosulfate solutions set forth in the prior artin the plating operation suffered from instability, and the processesemploying such solutions could only be operated at low current densitiesand within low-temperature limits.

The main shortcoming of the prior art plating solutions employingthiosulfate has been the instability of the electroplating solution.Gernes indicated in T rans. Electrochem. Soc. 77, 177 (1940) thatspontaneous decomposition of the bath takes place at temperatures aboveabout 50 C. Furthermore, Gernes disclosed that the limiting currentdensity was that at which a loose, black precipitate probably Cu,S" wasformed at the cathode which caused a turbid solution. The currentdensity employed by Gernes was not more than about 10 amperes per squarefoot.

The novel electroplating bath of this invention comprises an aqueoussolution containing (1) soluble copper in the equivalent of about 0.2 to0.5 molar concentration, (2) at least one alkali metal thiosulfateselected from the group consisting of sodium, potassium, and lithiumthiosulfate in about 0.4 to about L50 molar concentration, (3) at leastone alkali metal compound selected from the group consisting of sodium,potassium, and lithium sulfite and sodium, potassium and lithiumbisulfite, in about 0.1 to about 0.5 molar concentration, and (4) atleast one organic brightener in an amount of from about 0.005 to about0.25 molar concentration; the pH of the bath being maintained at about6.2 to about 7.5. Preferably, the soluble copper is derived from cuprousoxide, the alkali metal thiosulfate is sodium thiosulfate and the alkalimetal compound is sodium sulfite.

Surprisingly, it has been found that the solution instabilityexperienced by others when utilizing thiosulfate copper-plating bath canbe overcome and even reversed through the aeration of the novel platingbath of this invention during the plating operation. It is believed thatthe Cu,S formed is oxidized and converted to 5,0; by combining with thesulfite present in the bath. Strong aeration such as used inair-agitated plating baths, is not necessary. A small amount of airintroduced through a sparger, an open tube or by other suitable means ata location substantially below the surface of the solution in theplating tank is satisfactory. The exact amount of air introduced is notcritical but should be several times that required to theoreticallyeffect the oxidation reaction. Air agitation as used in the best ofmodern plating practice does have the advantage of increasing thecurrent density range and such agitation may be employed, if desired. Ithas been found that the solution stability of the novel electroplatingbath of this invention is maintained even on prolonged boiling and withcurrent densities above amperes per square foot cathodic.

Although good color and brightness were claimed by prior workers in theart such coatings would be unacceptable by today's standards for brightcopper plating. Attempts to produce a true copper color using thesolutions of the prior art workers yielded only grayish-brown depositswith no specular reflectivity. it has been found that the addition of asmall amount of certain cationic heterocyclic nitrogen compounds to theelectroplating 'bath yields a copper coating having specular brightness.Heterocyclic nitrogen compounds useful as brighteners in the novelelectroplating bath of this invention include compounds having from oneto two inclusive heterocyclic rings in which each of the said rings hasbetween five and six members inclusive including at least one N atom andwherein the substituents on the carbon atoms of the said heterocyclicrings are selected from the group consisting of H, NH, and alkyl of fromone to 10 carbon atoms as exemplified by methyl, ethyl, propyl,isopropyl, butyl, isoarnyl, hexyl, isohexyl, octyl, nonyl, decyl, etc.

An especially useful group of heterocyclic nitrogen containing compoundsemployed in the electroplating bath compositions of this inventionincludes pyridine, picoline, bipyridine, quinoline,2,7-dimethylquinoline, imidazole, 2- methylimidazole, benzimidazole,3-amino-l ,2,4-triazole, 5- aminotetrazole, 3,5-dimethylpyrazole,2,5-dimethylpiperazine, indole and nicotine.

Not all heterocyclic nitrogen compounds are suitable for use asbrighteners in this invention. For example, no brightening effect wasachieved with heterocyclic nitrogen compounds having carboxyl groups,such as 3-carboxypyridine, or with compounds having sulfonate, hydroxy,or keto substituent groups.

It has been found that the efficiency of the heterocyclic nitrogencompounds (brighteners) varies somewhat with the temperature of theplating bath employed. For example, pyridine exhibits a maximumbrightening effect at about 80 F. while with nicotine this is achievedat about 135 F. It is to be understood that compounds such as nicotinecan be added as the acid salt. For example, nicotine can be introducedas nicotine sulfate ((C I-I N,),-H,SO or as the hydrochloride ((C,I-I,,NBH2),-2HCl). The compound 3-amino-l,2,4-triazole,

in addition to being an excellent brightener, possesses the uniqueproperty of preventing anodic polarization when utilized in the novelelectroplating bath of this invention. It has been found that at aconcentration of 5 grams per liter (0.06M), the anode can be operated atfrom I0 to about 120 amperes per square foot at 100 percent efficiency.

PLATING BATH COMPOSITION For maximum cathodic efficiency it is desirableto keep the copper content of the solution as high as possible. In thenovel plating bath of this invention the copper-content of the solutionis present as a complex with the thiosulfate radical. When employingcuprous oxide as a source of copper, the complexing ratio is 2 moles ofsodium thiosulfate for each mole of cuprous oxide added as a minimumand, desirably, the plating bath will contain about 2.5 moles of sodiumthiosulfate for each mole of cuprous oxide present. The concentration ofcopper in the electroplating bath solution will be limited by thesolubility of alkali metal thiosulfate used which with the otheringredients will be somewhat more than about 1.5 molar concentration.The alkali metal sulfite is used to prevent atmospheric oxidation of thethiosulfate present in the bath composition. A minimum concentration ofabout 0.04 molar is desirable although concentrations of from about 0.0]to about 0.5 molar can be employed satisfactorily.

The copper present in the plating bath is most economically added ascuprous oxide although cuprous sulfite or cuprous chloride can beutilized. Likewise, it is also possible to employ a divalent coppercompound, such as cupric sulfate, cupric chloride, etc., as a source ofcopper in the plating bath. In such instances, the copper will bereduced to the monovalent form by the action of the alkali metal sulfiteand the quantity of the sulfite should be increased to allow for thisreduction if divalent copper compounds are employed.

The organic brighteners are all effective over a wide range of about0.001 to about 0.25 molar concentration. A preferred range of theheterocyclic nitrogen brighteners is from about 0.005 to about 0.20molar. The compound 3- amino-l,2,4-triazole which functions not only asa brightener but as an anode depolarizer as well is most effectivelyemployed at concentrations ranging from about 0.03 to about 0.15 molarwhen anode depolarization is required. During the plating operation itis important to maintain the pH of the plating bath composition at about6.2 to about 7.5 and, preferably, from about 6.5 to about 7.1 with theaddition of sodium hydroxide or sulfuric acid as required.

PREPARATION OF THE PLATING BATH SOLUTION The aqueous plating bath itselfcan be prepared by a variety of ways. Preferably, the alkali metalthiosulfate is added to the aqueous copper solution. In the next step,the alkali metal sulfite, and the heterocyclic nitrogen compound areintroduced. The pH is then adjusted to about 6.2 to about 7.5 onaddition of potassium hydroxide, sodium hydroxide, or sulfuric acid andthe resulting solution is filtered to remove any insoluble residue whichmay be present.

Although satisfactory plating of copper of steel is achieved underordinary conditions, the best adhesion of the copper plate to steel isobtained by cathodic activation of the steel in about 1 N sulfuric acidor sodium bisulfate for about l to ID minutes or more at 50 to I00amperes per square foot and at a temperature of about 70 to about l00 F.

The electrolytic plating baths exemplifying the compositions of thisinvention are generally operated under conditions normally used in theindustry, the temperature of the baths ranging from about to about F.Current densities of 5 to about 50 amperes per square foot are used andthe bath is air agitated.

The following examples illustrate various embodiments of this inventionare to be considered not Iimitative:

EXAMPLE I The following aqueous plating bath composition was prepared:

Copper, added as cuprous oxide 0.5 molar Sodium thiosulfate L25 molarSodium sulfite 0.04 molar Nicotine 0.l molar Water As required pHadjusted to 6.8 with sulfuric acid.

In the above-described plating bath, a steel cathode was plated at 120C. at 2 amperes for 5 minutes while the bath was agitated with air. Theplated cathode exhibited a good copper color with specular brightnessutilizing current densities up to about 50 amperes per square foot.

EXAMPLE II In this example, 3-amino-l,2,4,triazole was added to theplating bath solution of example I in 2 concentration of 0.l molar and asteel cathode was plated under the same conditions as in example I. Atthe end of the 5-minute plating period, the plated cathode exhibited agood copper color with high specular brightness at current densities upto about 50 amperes per square foot and, in addition, the anode wasclean whereas in the previous example a dark brown to black coated anoderesulted.

EXAMPLE III-XV A number of additional examples were performed in thesame manner asexample I utilizing a plating bath of the same compositionwith the exception that the nicotine brightener was replaced withanother brightener in each instance. The pertinent data relating tothese examples is shown below:

In each of the above-mentioned examples the cathode showed a good coppercolor with high-specular brightness at current densities of up to about50 amperes per square foot.

EXAMPLE XVl The steel cathode employed in this example was cleaned firstin a conventional alkaline cleaner following which it was cathodicallyactivated in 5 percent sulfuric acid for 3 minutes at v75 amperes persquare foot at 75 F. After plating 0.001 inch of copper on the cathodeutilizing the bath set forth in example ll the steel panel was bentrepeatedly until it broke but the copper plating on the steel remainedunimpaired condition indicating excellent adhesion.

EXAMPLE XVll The following is an example of a dry plating bathcomposition of this invention which has been found to give good results:

To prepare an electroplating solution from the above dry composition theingredients are dissolved in water to a concentration of about 2.5pounds per gallon. The pH is adjusted to 6.6-7.0 with sodium hydroxideand the resulting solution is filtered to remove any insoluble materialwhich may be present. If desired, the dry mix can be made up with onlythe three major ingredients and the additives introduced separately.

To illustrate the importance of air agitation and the use ofheterocyclic nitrogen compounds as brighteners in the process of thisinvention three comparative experiments were conducted. With a platingbath of the same composition as that employed in example I with theexception that it contained no nicotine, a brass cathode was plated in astandard cell (i.e., a Hull cell) at 120 F. at 2 amperes for 5 minuteswith no agitation and with no aeration. The cathode turned black withina few seconds and the solution became very turbid indicatingdecomposition of the plating bath. in a second experiment, a platingsolution of the same type as employed in example I with the exceptionthat nicotine was not present was utilized in plating a brass cathode at120 F. at 2 amperes for 5 minutes with air agitation of the bath (i.e.,aeration). The result was a dull, copper-plated brass cathode of poorcolor but no solution decomposition was experienced thus indicating theeffectiveness of aeration in the plating bath. In a third experiment,carried out in the same manner as the second experiment above, airagitation was replaced with nitrogen agitation and in this instance thebrass cathode exhibited a copper coating of poor color similar inappearance to that when air agitation was employed and the solutionagain became turbid.

The last experiment shows that nitrogen agitation does not preventsulfide formation and solution decomposition. Replacing the nitrogenwith air cleared the turbidity and restored normal plating conditionswithin a half hour.

What is l. is:

1. An electroplating bath comprising an aqueous solution of: (l solublecopper in the equivalent of about 0.2 to about 0.5 molar concentration,(2) at least one alkali metal thiosulfate selected from the groupconsisting of sodium, potassium and lithium thiosulfate in about 0.4 toabout l.5 molar concentration, (3) at least one alkali metal compoundselected from the group consisting of sodium, potassium, and lithiumsulfite, and sodium, potassium and lithium bisulfite, in about 0.01 toabout 0.0l molar concentration, and (4) at least one heterocyclicnitrogen compound in an amount of from about 0.005 to about 0.25 molarconcentration; the said heterocyclic nitrogen compound having from oneto two inelusive heterocyclic rings in which each of the said rings hasbetween 5 and 6 member inclusive including at least one N atom; whereinthe substituents on the carbon atoms of the said heterocyclic rings areselected from the group consisting of H,NH, and alkyl of from one to l0carbon atoms; and wherein the pH of the bath is about 6.2 to about 7.5.

2. The electroplating bath of claim I wherein the soluble copper isderived from cuprous oxide.

3. The electroplating bath of claim 1 wherein the said alkali metalthiosulfate is sodium thiosulfate.

4. The electroplating bath of claim 1 wherein the said alkali metalcompound is sodium sulfite.

5. The electroplating bath of claim I wherein the said heterocyclicnitrogen compound is selected from the group consisting of pyridine,picoline, bipyridine, quinoline, 2,7- dimethylquinoline, imidazole,2-methylimidazole, benzimidazole, 3-amino-l,2,4-triazole,S-aminotetrazole, 3,5- dimethylpyrazole, 2,5-dimethylpiperazine, indoleand nicotine.

6. The electroplating bath of claim 1 wherein the said heterocyclicnitrogen compound is nicotine.

7. The electroplating bath of claim I wherein the said heterocyclicnitrogen compound is 3-amino-l ,2,4-triazole.

8. The electroplating bath of claim 1 wherein the said soluble copper isderived from cuprous oxide, the said alkali metal thiosulfate is sodiumthiosulfate, the said alkali metal compound is sodium sulfite and thesaid heterocyclic nitrogen compound is nicotine.

9. A dry composition suitable for use in preparing an aqueous solutionfrom which copper may be electrodeposited on steel which comprises thefollowing ingredients in the following approximate percentages byweight:

Cuprous oxide l2 Sodium thio'sulfate, anhydrous 67.5 Sodium bisulfite,anhydrous I91) J-aminol ,2,4-triazole l.0 Nicotine sulfate 0.5

10. A method of electrodepositing copper on a steel cathode in whichcopper is electrodeposited from an elec troplating bath compositioncomprising an aqueous solution of l) soluble copper in the equivalent ofabout 0.2 to about 0.5 molar concentration, (2) at least one alkalimetal thiosulfate selected from the group consisting of sodium,potassium, and lithium thiosulfate in about 0.4 to about l.50 molarconcentration, (3) at least one alkali metal compound selected from thegroup consisting of sodium, potassium and lithium sulfite and sodium,potassium and lithium bisultite in about 0.01 to about 0.5 molarconcentration and (4) at least one heterocyclic nitrogen compound in anamount of from about 0.005 to about 0.25 molar concentration; the saidheterocyclic nitrogen compound having from I to 2 inclusive heterocyclicrings in which each of the said rings has between 5 and 6 membersinclusive including at least one N atom and wherein the substituents onthe carbon atoms of the said heterocyclic rings are selected from thegroup consisting of H,NH, and alkyl of from one to 10 carbon atoms,wherein the pH of said aqueous solution is maintained between 0.2 andabout 7.5 and wherein the electroplating bath is aerated during theelectroplating operation.

11. The process of claim 10 wherein the said alkali metal thiosulfate issodium thiosulfate.

12. The process of claim 10 wherein the said alkali metal compound issodium sulfite.

13. The process of claim 10 wherein the said heterocyclic nitrogencompound is selected from the group consisting of pyridine, picoline,bipyridine, quinoline, 2,7-dimemethylquinoline, imidazole,2-methylimidazole, benzimidazole, 3-aminol,2,4-triazole,S-aminotetrazole, 3,5-dimethylpyrazole, 2,5- dimethylpiperazine, indoleand nicotine.

14. The process of claim 10 wherein the said heterocyclic nitrogencompound is nicotine 15. The process of claim 10 wherein the saidheterocyclic nitrogen compound is 3-amino l,2,4-triazole.

16. The process of claim 10 wherein the said soluble copper is derivedfrom cuprous oxide, the said alkali metal thiosulfate is sodiumthiosulfate, the said alkali metal compound is sodium sulfite and thesaid heterocyclic nitrogen compound is nicotine.

17. The process of claim 10 wherein the said steel cathode prior tobeing plated is subjected to a metal cleaning cycle fol-

2. The electroplating bath of claim 1 wherein the soluble copper isderived from cuprous oxide.
 3. The electroplating bath of claim 1wherein the said alkali metal thiosulfate is sodium thiosulfate.
 4. Theelectroplating bath of claim 1 wherein the said alkali metal compound issodium sulfite.
 5. The electroplating bath of claim 1 wherein the saidheterocyclic nitrogen compound is selected from the group consisting ofpyridine, picoline, bipyridine, quinoline, 2,7-dimethylquinoline,imidazole, 2-methylimidazole, benzimidazole, 3-amino-1,2,4-triazole,5-aminotetrazole, 3,5-dimethylpyrazole, 2,5-dimethylpiperazine, indoleand nicotine.
 6. The electroplating bath of claim 1 wherein the saidheterocyclic nitrogen compound is nicotine.
 7. The electroplating bathof claim 1 wherein the said heterocyclic nitrogen compound is3-amino-1,2,4-triazole.
 8. The electroplating bath of claim 1 whereinthe said soluble copper is derived from cuprous oxide, the said alkalimetal thiosulfate is sodium thiosulfate, the said alkali metal compoundis sodium sulfite and the said heterocyclic nitrogen compound isnicotine.
 9. A dry composition suitable for use in preparing an aqueoussolution from which copper may be electrodeposited on steel whichcomprises the following ingredients in the following approximatepercentages by weight: Cuprous oxide 12 Sodium thiosulfate, anhydrous 67.5 Sodium bisulfite, anhydrous 19 .0 3-amino-1,2,4-triazole 1 .0Nicotine sulfate 0 .5
 10. A method of electrodepositing copper on asteel cathode in which copper is electrodeposited from an electroplatingbath composition comprising an aqueous solution of (1) soluble copper inthe equivalent of about 0.2 to about 0.5 molar concentration, (2) atleast one alkali metal thiosulfate selected from the group consisting ofsodium, potassium, and lithium thiosulfate in about 0.4 to about 1.50molar concentration, (3) at least one alkali metal compound selectedfrom the group consisting of sodium, potassium and lithium sulfite andsodium, potassium and lithium bisulfite in about 0.01 to about 0.5 molarconcentration and (4) at least one heterocyclic nitrogen compound in anamount of from about 0.005 to about 0.25 molar concentration; the saidheterocyclic nitrogen compound having from 1 to 2 inclusive heterocyclicrings in which each of the said rings has between 5 and 6 membersinclusive including at least one N atom and wherein the substituents onthe carbon atoms of the said heterocyclic rings are selected from thegroup consisting of H,-NH2 and alkyl of from one to 10 carbon atoms,wherein the pH of said aqueous solution is maintained between 0.2 andabout 7.5 and wherein the electroplating bath is aerated during theelectroplating operation.
 11. The process of claim 10 wherein the saidalkali metal thiosulfate is sodium thiosulfate.
 12. The process of claim10 wherein the said alkali metal compound is sodium sulfite.
 13. Theprocess of claim 10 wherein the said heterocyclic nitrogen compound isselected from the group consisting of pyridine, picoline, bipyridine,quinoline, 2,7-dimemethylquinoline, imidazole, 2-methylimidazole,benzimidazole, 3-amino-1,2,4-triazole, 5-aminotetrazole,3,5-dimethylpyrazole, 2,5-dimethylpiperazine, indole and nicotine. 14.The process of claim 10 wherein the said heterocyclic nitrogen compoundis nicotine
 15. The process of claim 10 wherein the said heterocyclicnitrogen compound is 3-amino-1,2,4-triazole.
 16. The process of claim 10wherein the said soluble copper is derived from cuprous oxide, the saidalkali metal thiosulfate is sodium thiosulfate, the said alkali metalcompound is sodium sulfite and the said heterocyclic nitrogen compoundis nicotine.
 17. The process of claim 10 wherein the said steel cathodeprior to being plated is subjected to a metal cleaning cycle followingwhich it is cathodically activated in a solution of a material selectedfrom the group consisting of sulfuric acid and sodium bisulfate for aperiod of about 1 to about 10 minutes at 50 to 100 amperes per squarefoot at a temperature of about 70* to about 100* F.
 18. The process ofclaim 17 wherein the said material is a 5 percent sulfuric acidsolution.